1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus and, more particularly, to positioning in the width direction of a sheet which is performed when the sheet with an image formed on one surface is reversed and is conveyed again to an image forming unit and an image is formed onto a reverse surface of the sheet.
2. Description of the Related Art
Hitherto, among image forming apparatuses such as an electrophotographic printer and the like, there is an apparatus constructed in such a manner that a sheet with an image formed on one surface is reversed and is conveyed again to an image forming unit, thereby forming an image onto a reverse surface of the sheet. Such an image forming apparatus has a sheet conveying apparatus for reversing the sheet with the image formed on one surface and conveying the sheet again to the image forming unit.
In such a conventional sheet conveying apparatus, in the case of conveying the sheet again to the image forming unit, there is a case where the sheet is obliquely moved during the conveyance and when the image is formed onto the reverse surface, the image is deviated from the sheet. This is because in the case of forming the image onto the second surface (reverse surface), since a conveying path to a position until the image is formed onto the sheet is longer than that for the first surface, the sheet is slightly deviated during the conveyance due to differences among eccentricity values and pressing forces of various rollers, a difference between resistance values of the conveying surfaces, or the like.
To prevent such a sheet deviation, therefore, it is necessary to adjust a position of the sheet so that the position of the image and that of the sheet coincide for a time interval until the image is formed onto the second surface after the image was formed onto the first surface. As such a sheet position adjusting method, for example, there is such a method whereby a reference surface is arranged in one end portion of a re-conveying path for conveying the sheet again to the image forming unit and the sheet is conveyed while pressing the sheet onto the reference surface, thereby performing the positioning in the width direction of the sheet (referred to as a lateral registration correction). For example, there is a technique disclosed in Japanese Patent Application Laid-Open No. 2000-233850.
FIGS. 10A, 10B, and 10C are top views illustrating a construction of the re-conveying path of the conventional sheet conveying apparatus having a lateral registration correcting portion for making the lateral registration correction of the sheet by the reference surface.
A lateral registration correcting portion 23 has: a reference guide 24 including a reference surface 24a; an diagonal feed roller pair 11A constructed by a diagonal feed roller 11 and a diagonal feed roller (not shown); and a conveyance lower guide 27. The diagonal feed rollers are arranged so as to face the reference surface 24a and have an inclination angle of about 7°. Each diagonal feed roller has a drum shape. Since the reference surface 24a is scraped by the sheet edge portion upon sheet passage, it is enhanced by arranging a plurality of reference pins 24h made of a metal.
The positioning operation of the lateral registration correcting portion 23 with such a construction will now be described.
As illustrated in FIG. 10A, a sheet S having the image formed on one surface is conveyed toward the lateral registration correcting portion 23 from conveying rollers 3g provided on the upstream of the lateral registration correcting portion 23. When the sheet S reaches the diagonal feed roller pair 11A soon, the sheet S is subsequently sandwiched between the diagonal feed roller pair 11A and conveyed. The sheet S is conveyed while being pulled toward the reference surface 24a side by the diagonal feed roller pair 11A.
Subsequently, when a rear edge of the sheet S goes away from the conveying rollers 3g, the sheet S receives a resistance of the conveyance lower guide 27 and rotates so that the sheet rear edge approaches the reference surface 24a side as illustrated in FIG. 10B. The sheet S is come into contact with the reference pins 24h by the rotation. The sheet rotates by its reaction force so that a sheet front edge approaches the reference surface 24a side.
Thus, the sheet S is aligned to a position along the reference surface 24a as illustrated in FIG. 10C. After that, the sheet S which has been positioned by the lateral registration correcting portion 23 as mentioned above is conveyed again to the image forming unit (not shown) through intermediate rollers 3d. 
As mentioned above, in the lateral registration correcting portion 23, the sheet S is rotated by the resistance of the diagonal feed roller pair 11A and the conveyance lower guide 27 and, thereafter, reversely rotated so as to be conveyed along the reference surface 24a while using the reference pins 24h as pivot points. According to such a lateral registration correcting method, since a conveyance distance to a position until the sheet is conveyed along the reference surface 24a after it went away from the conveying rollers 3g can be short, positioning (skew correction) efficiency is high.
However, in such conventional sheet conveying apparatus and image forming apparatus, since only one diagonal feed roller pair 11A has been provided, the position of the diagonal feed roller pair 11A in the sheet conveying direction has to be located within a range of the minimum length of the sheet to be conveyed. Therefore, it is necessary to set a distance between the diagonal feed roller pair 11A and the conveying roller 3g and a distance between the diagonal feed roller pair 11A and the intermediate roller 3d to be shorter than the minimum length of the sheet to be conveyed.
However, if the position of the diagonal feed roller pair 11A is set as mentioned above, the sheet whose length in the sheet conveying direction is shorter than the distance between the diagonal feed roller pair 11A and the conveying roller 3g cannot be conveyed. Therefore, in the case of conveying the sheets of many sizes including such a short sheet, it is necessary to provide a plurality of roller pairs into the re-conveying path.
For example, assuming that the minimum length of the sheet to be conveyed is set to the A5 size and the minimum feed size length is set to 210 mm, when a conveying path length between the conveying roller 3g and the intermediate roller 3d exceeds about 400 mm, it is necessary to provide at least two or more roller pairs into the re-conveying path.
However, for example, if two diagonal feed roller pairs 11A and 11B are provided in the re-conveying path as illustrated in FIG. 11, in the case where the sheet S is sandwiched between those two diagonal feed roller pairs 11A and 11B, the rotation of the sheet S that is caused by the conveyance resistance cannot be executed when the rear edge goes away from the conveying roller 3g. In other words, if the two or more diagonal feed roller pairs are provided as mentioned above, the foregoing lateral registration correction by the one diagonal feed roller pair 11A does not function.
Therefore, when the two or more diagonal feed roller pairs are provided, by moving the sheet S in parallel in the direction shown by an arrow B by the diagonal feed roller pairs 11A and 11B, the sheet S is come into contact with the reference surface 24a, thereby performing the lateral registration correction of the sheet S.
When the length of sheet S is long, since it is necessary to correct the skew before the front edge of the sheet reaches the intermediate roller 3d after it went away from the conveying roller 3g, it is necessary to increase nip pressures of the diagonal feed roller pairs 11A and 11B and increase a diagonal feed force adapted to make the sheet S approach in the direction of the arrow B. However, if the diagonal feed force is increased as mentioned above, for example, when the sheet S is thin (rigidity is small), since an approaching force in the direction shown by an arrow X illustrated in FIG. 12 is too large, the sheet S is come into contact with the reference surface 24a and bent. Thus, the sheet S is conveyed in the state where the position in the width direction for the reference surface 24a has been deviated. The lateral registration correction cannot be properly performed.
Since the approaching force in the width direction is too large, if the apparatus is used for a long time, the reference surface 24a is scratched by the sheet edge portion. Further, there is also a situation that the scratch on the reference surface 24a becomes a conveyance resistance to the sheet edge portion and a jam is caused by such a scratch.
In the case where the diagonal feed force is not increased in consideration of the bending of the thin sheet in the width direction and the scratch formed on the reference surface 24a, the sheet S has to be made to approach slightly by a vector for allowing the sheet to approach in the arrow B direction. Thus, a conveyance distance BL is necessary to allow the sheet to approach by a deviation amount BX of the sheet S in the width direction illustrated in FIG. 11.
That is, in the case of moving the sheet S in parallel by a plurality of diagonal feed roller pairs, there is a case where the lateral registration correction cannot be properly made in dependence on the rigidity of the sheet S. The conveyance distance necessary to make the sheet S approach becomes long in dependence on the length of sheet S.
If the size of sheet to be used is assumed to be, for example, the size in a range from A5 to the letter (legal) size, when the sheet is conveyed in a center reference manner, the sheet is conveyed in the state where it is away from the reference surface 24a by up to 34 mm (the letter width—A5) in the width direction. Even in such a case, the sheet S has to be made to approach the reference surface 24a. 
Also in such a case, since a skew correction amount is too large, for example, if the diagonal feed force for allowing the sheet S to approach in the arrow B direction is increased, the lateral registration correction cannot be properly made. If the sheet S of a long sheet feeding size, for example, the sheet S of a legal size is conveyed in the state where it is deviated in such a direction as to be away from the reference surface 24a, the sheet reaches the intermediate roller 3d before the lateral registration correction is finished.